Characteristics of the Beaufort Sea High

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MARK C. SERREZE AND ANDREW P. BARRETT National Snow and Ice Data Center, Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, Colorado

(Manuscript received 27 January 2010, in ﬁnal form 12 August 2010)

ABSTRACT

Characteristics of the Arctic Ocean’s Beaufort Sea high are examined using fields from the NCEP–NCAR reanalysis. At a 2-hPa contour interval, the Beaufort Sea high appears as a closed anticyclone in the long-term annual mean sea level pressure field and in spring. In winter, the Beaufort Sea region is influenced by a pressure ridge at sea level extending from the Siberian high to the Yukon high over northwestern Canada. As assessed from 6-hourly surface winds, the mean frequency of anticyclonic surface winds over the Beaufort Sea region is fairly constant through the year. While for all seasons a strong closed high can be interpreted as the surface expression of an amplified western North American ridge at 500 hPa, there is some suggestion of a split flow, where the ridge linked to the surface high is separated from the ridge to the south that lies within the main belt of westerlies. The Aleutian low in the North Pacific tends to be deeper than normal when there is a strong Beaufort Sea high. In all seasons but autumn, a strong Beaufort Sea high is associated with positive lower-tropospheric temperature anomalies covering much of the Arctic Ocean; positive anomalies are especially pronounced in spring. Seasons with a weak anticyclone show broadly opposing anomalies. A strong high is found to be a feature of the negative phase of the summer northern annular mode, the positive phase of the Pacific–North American wave train, and, to a weaker extent, the positive phase of the summer Arctic dipole anomaly and Pacific decadal oscillation. The unifying theme is that, to varying degrees, the high- latitude 500-hPa ridge associated with the Beaufort Sea high represents a center of action in each teleconnection pattern.

1. Introduction the Siberian coast, across the pole and into the North Atlantic, known as the Transpolar Drift Stream (Fig. 1) A prominent feature of the annual mean sea level (Thorndike and Colony 1982; Colony and Thorndike 1984). pressure (SLP) ﬁeld for the Arctic Ocean is an anticy- As is widely known, end-of-summer (September) Arc- clone centered north of Alaska, often referred to as the tic sea ice extent has declined over the past few decades. Beaufort Sea high (BSH). In the annual mean, the BSH September 2007 saw the lowest ice extent of the modern appears as a closed surface high embedded within a satellite era (Stroeve et al. 2008). The ice cover is also pressure ridge extending from northeastern Eurasia into thinning (Nghiem et al. 2006; Maslanik et al. 2007b; Kwok northwest Canada. The surface wind ﬁeld associated with and Rothrock 2009). Simulations from coupled global cli- the BSH, in conjunction with winds associated with the mate models used in the Intergovernmental Panel on Cli- trough of low pressure that extends from the Icelandic mate Change Fourth Assessment Report that include low into the eastern Arctic, largely controls the mean observed increases in atmospheric greenhouse gas con- circulation of the Arctic sea ice cover (Thorndike and centrations consistently show declining September ice Colony 1982). This circulation is characterized by the extent over the period of observations (Stroeve et al. anticyclonic Beaufort gyre and a transport of ice from 2007; Zhang and Walsh 2006). However, viewed as a group, simulated trends are conservative compared to observations (Stroeve et al. 2007). Many factors may be Corresponding author address: Mark C. Serreze, National Snow contributing to rapid ice loss. These include Arctic warm- and Ice Data Center, Cooperative Institute for Research in Envi- ronmental Sciences, Campus Box 449, University of Colorado ing linked to increased concentrations of black carbon Boulder, Boulder, CO 80309-0449. aerosols (Shindell and Faluvegi 2009); increased spring E-mail: [email protected] cloud cover, enhancing the downward longwave radiation

DOI: 10.1175/2010JCLI3636.1

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and location of the BSH and associated temperature anomalies are expressed with respect to the phase of atmospheric teleconnection patterns highlighted by other authors in the context of declining Arctic sea ice extent. Our analysis focuses on the period 1979–2008. While the NCEP–NCAR ﬁelds are available back to 1948, those from 1979 onward correspond to the modern satellite era and are therefore of higher quality. Use is made of surface winds, sea level pressure, 500-hPa height, and 925-hPa temperature. As shown in past studies (e.g., Serreze et al. 2009), basic circulation and tropospheric temperature features from NCEP–NCAR are very similar to those from other reanalyses. The 925-hPa temperatures are preferred over 2-m temperatures, which are strongly inﬂuenced by the modeled surface energy budget.

2. Background Interest in links between declining September sea ice extent and atmospheric circulation started in the mid- to late 1990s (e.g., Serreze et al. 1995; Maslanik et al. 1996) and thereafter grew quickly. Rigor et al. (2002) and Rigor FIG. 1. Annual mean SLP over the period 1979–2008 from the and Wallace (2004) provided important insight on in- NCEP–NCAR reanalysis with overlay of mean sea ice velocity vectors for 1979–2006 based on a combination of satellite and buoy fluences of the northern annular mode (NAM) in winter. data (http://nsidc.org/data/nsidc-0116.html). Ice motion is cm s21. The NAM, also known as the Arctic Oscillation, can be viewed as an oscillation of atmospheric mass between the Arctic and middle latitudes. It is in its positive phase when flux at the surface (Francis and Hunter 2006); and altered the zonally averaged surface pressure is high in mid- ocean heat transport (Polyakov et al. 2005; Shimada et al. latitude pressures and low in Arctic latitudes (Thompson 2006). However, as shown in numerous studies (e.g., and Wallace 1998, 2000). The North Atlantic Oscillation Rigor and Wallace 2004; Ogi and Wallace 2007; Wang (NAO), which relates to covariability in the strengths of et al. 2009; see section 2) variability in the atmospheric the Icelandic low and Azores high, is often viewed as the circulation, including the strength and location of the North Atlantic component of the NAM. From about 1970 BSH, has played an especially prominent role. through the mid-1990s, winter indices of the NAM shifted The present paper examines characteristics and vari- from negative to strongly positive. Rigor et al. (2002) show ability of the BSH, using data from the National Centers that as the winter NAM shifted toward the positive state, for Environmental Prediction–National Center for Atmo- there was a retreat of the BSH to the southern Beaufort Sea, spheric Research (NCEP–NCAR) Reanalysis I (Kalnay a more cyclonic motion of ice, and an enhanced transport et al. 1996). It is motivated by recognition that while dif- of ice away from the Siberian and Alaskan coasts, fostering fering points of view have developed regarding the role of openings in the ice cover. While open water in coastal atmospheric circulation anomalies on sea ice conditions areas quickly refroze in response to low-surface air tem- (see section 2), they can find some common ground through peratures, these regions were nevertheless left with an recognition that the BSH projects to varying degrees onto anomalous coverage of young, thin ice in spring, especially several atmospheric modes. We stress that we are not con- prone to melting out in summer. The strongly positive ducting a study of atmosphere–sea ice interactions. Our NAM phase characterizing the period 1989–95 saw strong focus is rather on variability of an atmospheric feature transport of thick, multiyear ice out of the Arctic and into recognized as highly relevant to the sea ice cover. the North Atlantic through the Fram Strait. While the Section 2 provides an overview of known links between NAM subsequently regressed to a more neutral state, the sea ice and the BSH. After evaluating mean seasonal ex- sea ice system may still have memory of these thinning pressions of the BSH in section 3, attention turns to char- processes (Rigor and Wallace 2004). acteristics of the large-scale midtropospheric circulation Other studies have focused on aspects of the summer that favor a strong or weak surface high (section 4). These circulation, and it is in this season that impacts of vari- analyses provide context for assessing how the strength ability in the BSH are especially prominent. Ogi and

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FIG. 2. Fields of (a) SLP, (b) SLP anomalies and (c) 925-hPa temperature anomalies averaged for summer (June– August) 2007. Anomalies are with respect to 1979–2008 means. The 925-hPa level gives a more useful assessment of lower-tropospheric warmth over the Arctic Ocean than does the surface temperature, which in summer over the ocean is strongly constrained by the melting sea ice cover.

Wallace (2007) find that years with a low September sea (2000), in which the NAM is based on a single EOF anal- ice extent tend to be preceded by anticyclonic summer ysis of geopotential height fields for all calendar months. (July–September) anomalies in the SLP field over the While by their approach, the NAM is most strongly ex- Arctic Ocean, with the core of the anomaly centered at pressed in winter, the NAM as defined by Ogi et al. (2004) about 858N, 2108E, hence somewhat north of the location is strong throughout the year, albeit with seasonal dif- of the BSH in the mean annual field (Fig. 1). Years with ferences in structure, and with the summer center of high September ice extent have the opposing anomaly action lying over the central Arctic Ocean. pattern. They view the atmospheric link largely in terms of The record September sea ice minimum of 2007 fos- Ekman drift in the marginal seas, with cyclonic winds tered a series of papers focusing on the summer atmo- leading to sea ice divergence, spreading ice over a larger spheric circulation. The key feature of summer 2007 was area, and anticyclonic winds promoting ice convergence, an unusually strong Beaufort Sea high, paired with un- compacting the ice into a smaller area. The anticyclonic usually low pressure over northern Siberia (Figs. 2a,b). anomaly pattern is also linked to positive anomalies in Resulting southerly winds between the pressure anomaly surface air temperature over the regions of reduced sea ice. centers promoted transport of ice away from the coasts of The anticyclonic–cyclonic SLP anomaly pattern is, in turn, Siberia and Alaska toward the North Pole (Ogi et al. 2008) similar to the pattern of the summer NAM as defined by as well as strong melt in the East Siberian and Chukchi Ogi et al. (2004). In their analysis, the NAM is defined se- Seas associated with the strongly positive air temperature parately for each calendar month through an empirical anomalies in this region (Fig. 2c) (Stroeve et al. 2008). orthogonal function (EOF) analysis of NCEP geopotential Unusually clear skies in the vicinity of the strong Beaufort height fields from 1000 to 200 hPa, poleward of 408N. This Sea high may have enhanced surface and basal melt (Kay contrasts with the approach of Thompson and Wallace et al. 2008), although the importance of cloud forcing has

Unauthenticated | Downloaded 10/07/21 01:40 AM UTC 162 JOURNAL OF CLIMATE VOLUME 24 been questioned (Schweiger et al. 2008). The pattern also led to a strong surface pressure gradient across Fram Strait (between Greenland and Svalbard) enhancing wind- driven transport of sea ice out of the Arctic Ocean and into the North Atlantic (Wang et al. 2009). The general view is that while this atmospheric pattern was key in driving rapid summer ice loss, its effectiveness was enhanced by the extensive coverage of thin, first-year ice in spring 2007—a reflection of the ongoing thinning of the ice cover noted earlier (e.g., Ogi et al. 2008; Stroeve et al. 2008). A similar, albeit less well-developed, atmospheric pattern dominated during the summer of 2008, as well as for most of the summer of 2009. Wang et al. (2009) performed an EOF analysis of seasonal mean SLP for the region north of 708N. They define the first EOF mode as the NAM, which has a seasonal structure very similar to that shown by Ogi et al. (2004). The second mode, termed the Arctic dipole anomaly (DA), has two centers of action of opposing sign—a pattern that has been remarked upon in various contexts in past studies (e.g., Overland and Wang 2005; Maslanik et al. 2007a; Stroeve et al. 2008). Focusing on the summer season, they conclude that while the NAM largely affects ice extent through Ekman drift processes, as discussed by Ogi and Wallace (2007), the DA, with centers of action centered over the northern Beaufort Sea and the Kara Sea, is associated with an anomalous meridional wind pattern. The positive DA phase, characterized by a strong, northward- shifted BSH and negative anomalies centered over the Kara Sea, leads to an anomalous wind component blowing across the Arctic Ocean that favors flushing of sea ice out of the Arctic Ocean through Fram Strait and, while not discussed in that paper, promotes warm southerly wind FIG. 3. Fields of (a) 500-hPa height and (b) anomalies from 408N anomalies over the East Siberian Sea. In this framework, to the pole, averaged for June through August 2007. Anomalies are the record seasonal sea ice minimum of September 2007, with respect to 1979–2007 means. the second-lowest minimum of September 2008, and the extreme minimum of September 2005 are consistent with of the summer dipole anomaly pattern, is seen to be lo- the combination of a positive summer mode of the DA and cated just downstream of an anomalously strong 500-hPa a negative mode of the summer NAM. This combination ridge favoring anticyclonic vorticity advection increasing was especially prominent in summer of 2007, with its ef- with height and downward motion. This 500-hPa pattern fects enhanced by the anomalous spring coverage of thin is viewed by L’Heureux et al. (2008) as an expression of first-year ice. Wang et al. (2009) also present evidence that an extreme positive phase of the Pacific–North American the positive DA with its strong BSH tends to promote the (PNA) wave train—three standard deviations above the import of warm ocean waters from the Pacific through the 1950–2007 summer mean (July to September in their Bering Strait favoring melt, and that this effect can also analysis). Recall that the winter PNA index describes help to explain the record 2007seaiceminimumand the amplitude of the wave train spanning the North Pa- possibly that of other recent summers. cific Ocean (mean trough) through northwestern North L’Heureux et al. (2008) provide yet another perspective America (mean ridge) to northeastern North America on summer 2007 and the role of the BSH. This is best (mean trough) (Wallace and Gutzler 1981; Barnston and discussed with the aid of Fig. 3, which provides the sum- Livezey 1987). As discussed by L’Heureux et al. (2008), mer 2007 mean 500-hPa height and anomaly fields down the summertime PNA wave train tends to be shifted to 408N. The anomalous mean BSH of summer 2007 (Fig. north of its winter position, and with a ridge located in the 2a), which in the framework of Wang et al. (2009) is part Beaufort Sea region.

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To summarize: 1) Variability in the high-latitude at- because of problems in reducing surface pressures over the mospheric circulation is identified as a contributor to the cold, high-elevation ice sheet to sea level (Serreze et al. observed downward trend in September sea ice extent and 2001). The spatial pattern of anticyclonic wind frequency recent extreme seasonal minima; 2) while this variability is very similar to the winter frequency pattern of closed has been examined in several frameworks leading to dif- high pressure cells as shown by Serreze et al. (1993), based fering interpretations, a common thread between differ- on an automated detection algorithm applied to daily SLP ent studies is that an important role is played by variability fields for 1952–89. It follows that regions of anticyclonic in the strength and location of the BSH. The following wind frequency maxima stand out as a relative minima in sections assess variability in the BSH and relationships the frequency of cyclonic winds. The cyclonic wind fre- with variability in the Arctic-wide and larger Northern quency pattern in Fig. 4b is similar to the frequency pat- Hemisphere circulation. tern of closed lows at the surface shown by Serreze et al. (1993, 1997), Zhang et al. (2004), and others. High frequencies in the North Atlantic are the expression of cy- 3. Mean seasonal cycle clone activity along the primary North Atlantic storm We start by examining mean seasonal cycles of anticy- track. Baffin Bay, by contrast, is known as a region where clonic and cyclonic surface winds across the Arctic, how cyclones tend to stall and dissipate, hence the particularly conditions across the Arctic compare to those in the high winter frequencies in this region (locally .80%). The Beaufort Sea region, and how spatial patterns in frequen- spatial patterns of anticyclonic and cyclonic flow are con- cies of cyclonic and anticyclonic flow are reflected in the sistent with the mean seasonal distribution of sea level distributions of SLP. We use the frequency of negative pressure. At a 2-hPa contour interval adopted for Fig. 4c, relative vorticity as an index of the occurrence and strength there is no closed Beaufort Sea high in the winter mean of the BSH. Relative vorticity, in representing a measure of field. Rather the East Siberian, Chukchi, and Beaufort rotation of the wind field, is a better index of anticyclonic Seas are overlain by a ridge of high pressure. The high- conditions than sea level pressure. est pressures over Eurasia are part of the well-known Surface u and v winds from the NCEP–NCAR rean- Siberian high. A closed BSH does appear when the data alysis were used to compute relative vorticity every 6 hours are mapped using a finer 1-hPa contour interval. at all grid locations poleward of 608Nforthetimeperiod Corresponding maps for spring (March–May; Fig. 5) 1979–2008. The 6-hourly NCEP–NCAR data are provided document a reduction in the frequency of anticyclonic on a 2.5832.58 latitude–longitude grid. The panels winds over Siberia compared to winter, with the Beaufort constituting Fig. 4 depict the winter season (December Sea region still appearing as a relative peak (50%–60%). through February) percent frequency of negative (anti- Cyclonic wind frequency is reduced compared to winter cyclonic) (Fig. 4a) and positive (cyclonic) relative vor- in the Atlantic sector. In accord with weakening of the ticity events at the surface by grid cell (Fig. 4b), and the Siberian high and its poleward extension, the mean SLP mean SLP (Fig 4c). For a given grid location, anticyclonic field for spring shows a pronounced closed Beaufort Sea (negative vorticity) frequency is defined as the percent- high with a peak central pressure of about 1022 hPa. age of all 6-hour time periods (4 analyses per day times Patterns for summer are quite different from winter the 90 days constituting the winter season times the 30 and spring (Fig. 6). The local maximum in anticyclonic years analyzed) that the relative vorticity was more nega- winds in the Beaufort Sea seen in winter and spring is tive than the lowest 25th percentile of all negative vorticity shifted to the south. The distribution of anticyclonic wind events for winter, as assessed for the region poleward of maxima is consistent with the frequency maxima of closed 608N. Cyclonic vorticity frequency was calculated in the summer highs depicted in the earlier analysis of Serreze same way using the number of positive vorticity events et al. (1993). The area around the pole shows up as a weak with magnitude larger than the 25th percentile value of relative maximum in cyclonic winds. The positive and all positive vorticity events. The screening eliminates the negative vorticity frequency patterns are expressed in the weak vorticity cases. All results that follow are based on mean SLP field as a weak (1015 hPa) Beaufort Sea high these thresholds. Maps based on different thresholds (e.g., (closed only for a 1-hPa contour interval) with a more the 50th percentile) have very similar spatial patterns. limited spatial extent compared to its spring counterpart, While the frequency of anticyclonic events in the paired with an area of weak mean low pressure centered Beaufort Sea region stands out as a relative peak (50%– just off the North Pole. This mean low has long been 60%), frequencies are higher over northeastern Siberia. recognized (e.g., Reed and Kunkel 1960) and reflects the Fairly high frequencies are also found over parts of north- summer maximum in cyclone activity in this region. These ern Canada. Peak frequencies over the central Greenland systems variously form within the central Arctic Ocean ice sheet (.90%) should be viewed with extreme caution or migrate into the region from Eurasia (Serreze and

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FIG. 4. Winter (December–February) ﬁelds of the percent frequency of (a) anticyclonic and (b) cyclonic winds at the surface from 6-hourly wind ﬁelds, and (c) mean SLP (2-hPa contour interval), all based on data for the period 1979–2008.

Barrett 2008). Manifesting an equivalent barotropic struc- a ridge (a closed high only apparent at a 1-hPa contour ture, the weak mean low lies almost directly underneath interval) and the Siberian high has started to rebuild. the center of the summer 500-hPa polar vortex (not Fig. 8 provides monthly means and standard devi- shown). ations of negative and positive vorticity frequency, along The maps for autumn (Fig. 7) document the transition with monthly mean SLP based on aggregating data over back toward winter conditions. Peak frequencies of an- the region bounded by latitudes 72.58–80.08N and longi- ticyclonic winds in the Beaufort Sea (again 50%–60%) tudes 180.08–225.08E (see Fig. 7c). This region encom- have shifted north of their summer location, and the passes the center of the BSH region as it appears in the frequency of cyclonic winds has increased in the Atlantic vorticity frequency maps. Figure 9 shows, for the same sector. The Beaufort Sea high region is again part of region, monthly means, 5th, 25th, 75th, and 95th percentile

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FIG. 5. As in Fig. 4, but for spring (March–May). values of vorticity magnitude. While there is only a weak central Arctic (including the BSH region) from winter seasonal cycle in anticyclonic frequency in the BSH re- into spring is linked to a poleward mass transport from gion, there is a 10-hPa range in mean SLP, from a maxi- Eurasia. An increase in equatorial transport over the mum of 1021 hPa in March to a minimum of 1011 hPa in Canadian Arctic Archipelago in May and June is mani- August. The August pressure minimum manifests the fested as a decrease in surface pressure over the Arctic August maximum in the frequency of positive vorticity Ocean into summer. This pattern then reverses in au- events (which tend to correspond to low-pressure sys- tumn, with transport from the Canadian Arctic Archi- tems), the southward shift in summer of maximum vor- pelago into Eurasia. ticity frequency (cf. Figs. 4a–6a), and a more general It is useful to contrast March (pressure maximum) and mass transfer out of the Arctic Ocean in summer. The August (pressure minimum) with respect to the distribu- latter issue was examined by Cullather and Lynch (2003). tion of omega (vertical motion). The plots in Fig. 10 show They show that the increase in surface pressure over the omega (Pa s21, positive values meaning downward

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FIG. 6. As in Fig. 4, but for summer (June–August). motion) along latitude 758N from longitudes 908E east- variability in the BSH and the location and strength ward to 908W and vertically from 1000 to 100 hPa. The of the midtropospheric flow are discussed in the next March SLP maximum in the BSH region is supported by section. a prominent region of downward (positive) omega cen- 4. Interannual variability and composite analyses tered along approximately 1508W, peaking at about 600 hPa (defining the approximate level of nondivergence), Standardized anomalies in the frequency of anticyclonic consistent with the location of the BSH and its associ- vorticity events over the Beaufort Sea subregion by year ated divergent surface wind field ahead of a mean ridge and season are plotted in Fig. 11. Anomalies are com- at 500 hPa. The BSH pressure minimum in August is as- puted with respect to frequencies for the period 1979– sociated with a much weaker region of downward omega, 2008. There are no statistically significant (at the 10% with the maximum shifted east to about 1208W, and level) trends in any season. Anomalies have been positive peaking closer to the surface. Links between interannual for the last five years of the analyzed record (2004–08) in

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FIG. 7. As in Fig. 4, but for autumn (September–November). The box plotted in (c) defines the region for which data are aggregated to assess monthly and interannual variability in the strength of the BSH. autumn and for four of the past five years in summer. The correlations (.0.6) are located over the Beaufort Sea. In most positive frequency anomalies for both summer and winter, spring, and autumn, vorticity in the BSH region is autumn occurred in 2007. also negatively correlated with SLP in the North Pacific Fig. 12 shows maps of correlation coefficients between corresponding broadly to the Aleutian low region. In the standardized anomalies in the frequency of anticy- spring, a strong BSH also relates to lower pressures over clonic vorticity events in the BSH region as we have de- western Eurasia. In summer and autumn, pressures in the fined it and fields of seasonal mean sea level pressure for BSH region are furthermore inversely correlated with the period 1979–2008. Coefficients are only shown where those over Eurasia and (in summer) Europe and the Sea they are significant at the 5% level. Reinforcing the val- of Okhotsk. idity of the frequency of anticyclonic relative vorticity as Variations in the strength of the BSH are associated an index of the strength of the BSH, strong positive with distinctive signatures in large-scale atmospheric

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FIG. 8. Monthly means for the region encompassing the climatological center of the Beaufort Sea high (see Fig. 7c) of (a) SLP and the percent frequency of negative relative vorticity, and (b) SLP and the percent frequency of positive relative vorticity. SLP corresponds to the dotted lines. Vorticity frequency is shown in the solid lines, with shading encompassing the 61 standard de- FIG. 9. Monthly distribution of (a) negative and (b) positive rel- viation of monthly frequencies. Results are based on 6-hourly ative vorticity magnitude aggregated for the region encompassing analyses for the period 1979–2008. the climatological center of the Beaufort Sea high. The horizontal line across each box is the mean, and the upper and lower bounds of circulation and temperature fields. The composite fields each box are the 75th and 25th percentile values. The dotted lines constituting Fig. 13 of SLP anomalies, 500-hPa height, extend to the 5th and 95th percentile values. anomalies of 500-hPa height, and anomalies of 925-hPa temperature summarize the situation for winter months 500-hPa flow (Figs. 13b,c). While viewed most simply as with a strong BSH. Strong BSH winter months are de- a high-latitude expression of an amplified western North fined as those for which the monthly negative vorticity American ridge, there is also suggestion of a regional split frequency for the BSH region falls into the highest 20% flow—note how the ridge with fairly tight height gradients of values for the 90 winter months included in the 30-yr linked to the anomalously strong BSH is separated by a (1979–2008) record. The composites therefore represent region of slack height gradients from the ridge to the south the mean of 18 months; anomalies are with respect to associated with the main belt of westerlies. Separation is winter means over the period 1979–2008. further evident in the much more pronounced 500-hPa Positive SLP anomalies in the Beaufort Sea (Fig. 13a), height anomalies linked to the ridge in the Beaufort Sea peaking at about 5 hPa, are associated with a closed an- region compared to the ridge at lower latitudes. Consis- ticyclone with a central pressure of 1022 hPa (not shown). tent with the correlation analyses just presented, the The strong BSH lies east of a pronounced ridge in the strong BSH at sea level and its associated 500-hPa ridge

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21 FIG. 10. Mean vertical motion (omega, Pa s , positive meaning downward motion) for March and August at 758N from longitudes 908E eastward to 908W and from 1000 to 100 hPa.

occur in conjunction with negative 500-hPa height and SLP anomalies over the North Pacific indicative of an anomalously deep Aleutian low. Linear correlations between the monthly BSH vorticity index and the minima of monthly mean pressures in the region 408–608Nand 1608E–1608W; an index of the Aleutian low strength used by Overland et al. (1999) further supports this in- verse relationship. Positive pressure anomalies are found in the Icelan- dic low region (again, see Fig. 12), and negative pressure anomalies extend across the northern North Atlantic. Consistent with the 500-hPa anomaly structure, 925-hPa temperatures are higher than normal over northwestern FIG. 11. Standardized seasonal anomalies (z scores) of negative North America and the western Arctic Ocean. Anomaly vorticity frequency aggregated for the region encompassing the patterns for the case of a weak BSH (not shown), based on climatological center of the Beaufort Sea high. the winter months for which the negative vorticity frequency fell into the lowest 20% of the distribution, largely mirror those seen in the positive composites; positive Patterns associated with a strong BSH in spring (Fig. 14) pressure anomalies extend across the northern North At- share some similar features with the winter expressions. As lantic and 925-hPa temperatures are cooler over western with winter, positive SLP departures over the Beaufort Sea Arctic Ocean and northwestern North America. There is (reflecting a closed anticyclone with a central pressure of no evidence of a regional split flow at 500 hPa and height 1024 hPa) are paired with an anomalously deep Aleutian gradients over the Arctic Ocean are quite slack. This low. Positive pressure anomalies are found in the Icelandic mirroring of anomaly patterns in composites for a strong low region. Negative SLP anomalies also characterize versus weak BSH is evident in all seasons. much of northern Eurasia (see Fig. 12). The amplified

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FIG. 12. Linear correlation coefficients between standardized seasonal anomalies of negative vorticity frequency for the region encompassing the climatological center of the Beaufort Sea high and fields of seasonal mean SLP. Only correlations significant at the 5% level are shown.

500-hPa ridge associated with the strong BSH, with its northern North Pacific. Similar to winter and spring, axis roughly along the date line, is again somewhat sep- however, the anomalously strong BSH can be associated arated from the amplified western North American ridge. with a 500-hPa ridge that is separated from the western While like winter when positive 925-hPa temperature North American ridge to the south. The notable feature anomalies are found over northwestern North America of the 925-hPa temperature anomaly field is the positive and the western Arctic Ocean, positive anomalies now departures covering much of the western Arctic Ocean. also cover almost all the plotted region north of 408N. Composite fields for autumn months with a strong BSH Plots for summer months with a strong BSH follow in are provided in Fig. 16. The dominant feature of the SLP Fig. 15. The dominant feature of the SLP anomaly field is field is the pairing of positive departures over the Beaufort the pairing of positive departures over the Beaufort Sea Sea with a stronger-than-normal Aleutian low in the North (linked to a closed anticyclone with a central pressure of Pacific. Note, however, that the SLP composite does not 1016 hPa) with a weaker-than-normal Icelandic low. Weak show prominent negative anomalies over eastern Eurasia, negative anomalies characterize lower latitudes and are as would be expected from the correlation analysis pre- somewhat stronger over eastern Eurasia and part of the sented in Fig. 12. The regional split flow at 500 hPa seen in

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FIG. 13. Fields for the region poleward of 408N of (a) SLP anomalies, (b) 500-hPa height, (c) 500-hPa height anomalies, and (d) 925-hPa anomalies averaged for winter months for which the Beaufort Sea high was strong. winter, spring, and summer is again present. Interestingly, on the top and bottom 20% of the normalized monthly 925-hPa temperature anomalies over the domain from index values for each pattern. Discussion focuses on the 408N to the pole, including the Arctic Ocean, are mostly summer and winter seasons. negative. The NAM analysis uses monthly index values based on the work of Ogi et al. (2004) (http://wwwoa.ees.hokudai. ac.jp/people/yamazaki/SV-NAM/index.html). 5. Links to atmospheric teleconnections Recall that they define the NAM separately for each Summarizing the preceding discussion, a strong BSH in calendar month. This contrasts with the approach of all seasons is linked to a pronounced ridge at 500 hPa. Thompson and Wallace (2000) who used a single EOF Except for autumn, a strong BSH is associated with pos- analysis for all calendar months. We calculate a monthly itive lower-tropospheric temperature anomalies covering DA index following the approach of Wang et al. (2009) for much or all of the Arctic Ocean. Seasons with a weak their seasonal DA index, but using monthly rather than anticyclone show broadly opposing anomalies. seasonal fields. This ensures that composites comprise the To build on these findings, we consider expression of same number of cases for all indices. Recall that Wang the BSH with respect to the phase of atmospheric tele- et al. (2009) defined the DA as the second principal com- connection patterns cited as having links with the ob- ponent of the seasonal mean SLP field north of 708N. The served decline in Arctic sea ice extent. These include the first principal component is taken to be the NAM and has NAM, the Arctic DA, and the PNA wave train. For com- a seasonal structure very similar to that shown by Ogi et al. pleteness, we also examine links with the Pacific decadal (2004). For the monthly DA index, following L’Heureux oscillation (PDO), which is known to have prominent cli- et al. (2008) (see below), the month of interest as well as mate signals in the North Pacific sector. We employ the the previous and following months are included in the same approach as above, except that composites are based EOF analysis. The first and second EOFs and time series

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FIG. 14. As in Fig. 13, but for spring months. of principal components for the monthly analysis compare The PDO (http://jisao.washington.edu/pdo/PDO.latest) well with the seasonal DA index time series of Wang et al. is defined as the leading principal component of monthly (2009). anomalies of sea surface temperature for the northern However, the DA index used here needs to be treated Pacific Ocean (north of 208N) (Mantua et al. 1997). The with caution. The EOF analysis is performed over a lim- PNA time series, obtained from the NCEP Climate Pre- ited area (north of 708N), which can produce EOFs with diction Center (http://www.cpc.ncep.noaa.gov) is the same patterns that are not related to underlying patterns in as that used by L’Heureux et al. (2008) in their analysis the original data (Buell 1979; Wilks 2006). The first four of atmospheric circulation for the summer of 2007. A de- EOFs for each of the monthly analyses display patterns tailed description of the methods used to calculate the similar to those first described by Buell (1979); often re- PNA is given at http://www.cpc.ncep.noaa.gov/data/ ferred to as one fried egg, two fried eggs, etc. Furthermore, teledoc/telepatcalc.shtml. Briefly, the 10 leading un- inspection of the eigenvalue spectra for each of the months rotated EOFs are calculated for each calendar month and variances accounted for by the second and third EOFs using standardized fields of monthly mean 500-hPa height show that there is little separation between the second and anomalies for the region poleward of 208N for the period third eigenvalues. In the monthly analyses, the leading January 1950 to December 2000. Instead of using data for EOFs account for between 37% and 65% of the variance. just the month of interest, data for the two neighboring The second EOFs interpreted here as the DA account for months are also used. For example, the EOFs for July are between 10% and 22%. The third EOFs account for be- calculated using fields for June, July, and August. The 10 tween 7% and 16%. Limited separation between eigen- leading EOFs for each month are then subjected to a values indicates that the associated EOFs may represent varimax rotation to produce 10 rotated EOFs and indices, an arbitrary mixture of the variance of the original data one of which is the PNA. The NAO is one of the other (Wilks 2006). Similar ‘‘fried egg’’ patterns and eigenvalue indices. The PNA calculated using this method is viewed spectra are found for seasonal analyses as well. as an improvement over the three-point-based index

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FIG. 15. As in Fig. 13, but for summer months.

(Wallace and Gutzler 1981) because it has better con- along the Eurasian coast and weak positive anomalies tinuity between months and uses information from the over the western side of the Arctic; a pattern that, as dis- whole flow field (L’Heureux et al. 2008). cussed by Wang et al. (2009), promotes transport of ice out Composite SLP anomaly fields for summer months of the Arctic Ocean via the Fram Strait. The PNA com- with a strong BSH from the vorticity index appear in posite anomaly field is similar to the summer NAM pattern Fig. 17 along with corresponding fields for the negative in showing negative anomalies over the North Pacific and phase of the NAM, the positive phase of the DA, the positive anomalies in the BSH region, but, in the PNA, the positive phase of the PNA, and the positive phase of the Beaufort Sea anomaly is much weaker. The positive PDO PDO.ThecompositeanomalyfieldforthestrongBSH composite, by contrast, is associated with predominantly and the negative phase of the summer NAM are highly positive SLP anomalies over Arctic and subarctic latitudes, similar—the primary difference is that the positive SLP with a weak positive anomaly in the eastern BSH region anomalies over the Arctic Ocean are stronger and cover extending into the Canadian Arctic Archipelago. a larger area in the NAM composite. Of the 18 summer In summary, while a strong BSH based on the vorticity months represented in the strong BSH composite, 9 were index is most closely allied with the negative phase of the associated with a negative summer NAM index value. The NAM, there is some tendency for a stronger BSH during linear correlation between the two monthly index time the positive phase of the DA, PDO, and particularly the series for summer months is statistically significant at the PNA. Anomaly structures for the composites for the 5% level in June (20.58) and July (20.59) but not for positive-phase summer NAM, and the negative phase of August (20.24). Time series of monthly indices for the the DA, PNA, and PDO largely mirror those shown in NAM and BSH for June and July follow in Fig. 18. Fig. 17. This mirroring largely holds with respect to re- There is little similarity between the strong BSH maining discussion in this section. composite and the positive DA composite, for which the Turning to 500-hPa height fields (Fig. 19), the salient Arctic region is dominated by negative SLP anomalies feature of the NAM composite is the more pronounced

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FIG. 16. As in Fig. 13, but for autumn months. curvature in the streamlines in the Beaufort Sea sector as (e.g., Thompson and Wallace 1998, 2000). In further compared to the composites for DA, PNA, and PDO. This sharp contrast to summer, the negative NAM phase is is seen as a strong positive anomaly in 500-hPa heights linked to positive 925-hPa temperature anomalies over centered at about 858N and along the date line, just west of much of the Arctic Ocean. The poor relationship between the positive SLP anomaly (not shown). A weaker, albeit the NAM and BSH in winter is reflected in the linear still prominent, positive height anomaly is associated with correlation between the monthly indices for these patterns. the positive PNA composite at about the same latitude, Only January has a weak (20.37) but statistically signif- but slightly farther east. The negative NAM phase is as- icant correlation at the 5% level. While the positive PNA sociated with positive lower-tropospheric temperature phase in winter is dominated by positive high-latitude anomalies over the western Arctic Ocean, but of smaller temperature anomalies, the spatial structures in winter spatial extent than for the strong BSH composite or the and summer are quite different (Fig. 22). DA (Fig. 20). During positive extremes of the PNA, essentially all of the Arctic Ocean is covered by positive 6. Summary and discussion temperature anomalies. SLP anomaly patterns for the winter composites of the As assessed using 6-hourly fields from the NCEP– NAM, DA, PNA, and PDO bear little resemblance to NCAR reanalysis, the frequency of anticyclonic surface their summer counterparts (Fig. 21) and none depict a winds in the Beaufort Sea is fairly constant through the strong BSH. While the BSH is clearly allied with the NAM year. As a closed anticyclone (2-hPa contour interval) in in summer and, to a lesser degree, the PNA, the negative the climatological mean SLP fields, a BSH is present only NAM composite instead depicts a large-scale pattern of in the annual mean and in spring. In winter, the region compensating positive anomalies over all of the Arctic is instead influenced by a pressure ridge extending from and negative anomalies in middle latitudes. This struc- the Siberian high to the Yukon high over northwestern ture has, of course, been discussed in numerous studies Canada. The mean summer SLP field in the BSH region

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FIG. 17. SLP composite anomaly ﬁelds for the region poleward of 408N based on summer months for (a) a strong BSH, (b) negative phase of the NAM, (c) positive phase of the Arctic DA, (d) positive phase of the Paciﬁc North American wave train, and (e) positive phase of the PDO.

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maximum is allied with a tendency for the summer 500-hPa circumpolar vortex to contract and become broadly symmetric about the pole, and with development of a region of high-latitude baroclinicity. Development of this high- latitude baroclinicity is driven at least in part by differential heating between the land and Arctic Ocean. The cold Arctic Ocean may also help to focus the center of the 500-hPa polar vortex over the pole. In turn, there is an eastward shift of the Urals trough, and the flow ahead of it becomes more zonal than in winter. Systems entering the central Arctic Ocean from the outside, or formed within the Arctic Ocean, migrate around the 500-hPa vortex and decay within the cyclone maximum region, or in close proximity (Serreze et al. 2001). However, the strength of the cyclone pattern is highly variable. When well developed, the 500-hPa circumpolar vortex is particularly strong and symmetric about the pole, with negative SLP anomalies centered over the pole and positive anomalies over middle latitudes. The BSH is weak. This tends to occur in conjunction with the positive FIG. 18. Time series of standardized monthly anomalies of fre- NAM phase. For summer months, when the cyclone quency of negative vorticity for the Beaufort Sea region (solid) and pattern is weakly developed, the 500-hPa circumpolar the NAM index (inverted, dashed) for June and July. The statistical significance of the correlation coefficients is shown in parentheses. vortex is weak and the flow is much more meridional, with a pronounced 500-hPa ridge over the northern Beaufort Sea and a strong BSH. This tends to occur in conjunction is rather flat. For all seasons, a pronounced closed high at with the negative phase of the NAM. the surface is linked to a pronounced ridge at 500 hPa. Composite analyses presented here, as well as results While viewed most simply as a high-latitude expression from other studies, indicate that a stronger (weaker) than of an amplified western North American ridge, there is average BSH in summer months may also attend the also the suggestion of a regional split flow, in that the ridge positive (negative) phases of the Arctic dipole anomaly, with fairly tight height gradients linked to the anomalously the Pacific–North American teleconnection, and the Pacific strong BSH is separated by a region of slack height gra- decadal oscillation. At least in a composite mean sense, dients from the ridge to the south associated with the main after the NAM, the strength of the BSH is most strongly belt of westerly winds. Separation is further evident in the linked to the phase of the PNA. Recall from earlier dis- much more pronounced 500-hPa height anomalies linked cussion that L’Heureux et al. (2008) linked the unusual to the ridge in the Beaufort Sea region compared to the circulation in summer 2007 that drove the record Septem- ridge at lower latitudes. In all seasons but autumn, a strong ber sea ice minimum to an extreme positive (three standard BSH is associated with positive lower-tropospheric tem- deviation) phase of the PNA. While the NAM was also perature anomalies covering much of the Arctic Ocean; negative for June and July of 2007 (index values of 20.81 positive anomalies are especially pronounced in spring. and 20.740), it was slightly positive in August (10.29). The There are no obvious temporal trends in the strength of DA was, in turn, a positive mode. the BSH as measured by the frequency of anticyclonic The unifying theme, which provides some common winds in the region. ground to understanding results from past studies that Variations in the strength of the summer BSH are have employed a variety of frameworks (NAM, PNA, clearly allied with the phase of the summer NAM. This is DA) to diagnose links between atmospheric circulation consistent with demonstrated links between the summer and sea ice conditions, is that to varying degrees, the NAM and high-latitude extratropical cyclone activity. In a high-latitude 500-hPa ridge associated with the surface climatological sense, the frequency of extratropical cyclone BSH represents a center of action in each teleconnection centers over the central Arctic Ocean has a distinct sum- pattern: it is the dominant center of action for the NAM, mer peak. Summer activity is greatest at about 858Nalong weaker and one of several centers for the PNA, and weaker the date line (Serreze and Barrett 2008). This basic pattern still for the DA and PDO. These differences in the strength has been recognized for many years (e.g., Dzerdzeevskii and location of the 500-hPa center of action are of course 1945; Reed and Kunkel 1960). Seasonal onset of the cyclone reflected in the composites in the location and amplitude of

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FIG. 19. As in Fig. 17, but for ﬁelds of 500-hPa height.

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FIG. 20. As in Fig. 17, but for ﬁelds of 925-hPa temperature anomalies.

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FIG. 21. SLP composite anomaly ﬁelds for the region poleward of 408N based on winter months for (a) a strong BSH, (b) negative phase of the NAM, (c) positive phase of the Arctic DA, (d) positive phase of the Paciﬁc North American wave train, and (e) positive phase of the PDO.

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FIG. 22. As in Fig. 21, but for ﬁelds of 925-hPa temperature anomalies.

Unauthenticated | Downloaded 10/07/21 01:40 AM UTC 1JANUARY 2011 S E R R E Z E A N D B A R R E T T 181 the SLP anomalies. This contrasts with winter, when there Ogi, M., and J. M. Wallace, 2007: Summer minimum Arctic sea ice are no clear relationships between the strength of the BSH extent and the associated summer atmospheric circulation. and any of the teleconnection patterns examined. This Geophys. Res. Lett., 34, L12705, doi:10.1029/2007GL029897. ——, K. Yamakazi, and Y. Tachibana, 2004: The summertime follows in that as viewed individually, none have a 500-hPa annular mode in the Northern Hemisphere and its linkage to center of action in the BSH region. the winter mode. J. Geophys. Res., 109, D20114, doi:10.1029/ 2004JD004514. Acknowledgments. This study was supported by NSF ——, I. G. Rigor, M. G. McPhee, and J. M. Wallace, 2008: Summer Grants ARC-090162 and ARC 0805821. retreat of Arctic sea ice: Role of summer winds. Geophys. Res. Lett., 35, L24701, doi:10.1029/2008GL035672. 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